Optical smoke detectors, also designated as photoelectric smoke detectors, are known to have a detection chamber in which a light source and a light receiver are located. The interior of the smoke chamber usually is protected from ambient light but includes smoke ports which feed into the dark smoke chamber via a labyrinth of smoke entrances. The light source and the light detector are arranged in the smoke chamber relative to each other in such a way that light emitted from the light source does not directly impinge on the light detector. Rather, the light detector will sense stray light reflected from smoke inside the smoke chamber and hence will generate different output values dependent on the presence or absence of smoke in the chamber. Examples of light sources are light emitting diodes, such as an IR diode or different color diodes. Examples of the light receivers are a photo detector diode and a phototransistor.
The evaluation of the output signal of the photo detector usually requires several analog circuits or specialized integrated circuits which, among others, are needed to improve the signal-noise-ratio (SNR), eliminate adverse effects from the inherent capacitance of the photo diode or other receiver and to amplify the rather weak signal emitted from the photo diodes when exposed to only weak light emittance. One reason why the light emitter will have only a relatively low luminous emittance is that smoke detectors are often used as stand-alone devices, powered by batteries, and need to be able to operate at very low power consumption so as to ensure a long service life.
The output signals of the photo detector hence are typically processed by using a trans-impedance amplifier where the output voltage of the photo detector is stabilized to a fixed voltage level by compensating the discharge current in the detector during light exposure with a charge current from an operational amplifier. By using the trans-impedance amplifier, the voltage at the light receiver is kept constant so that its inherent capacitance does not have to be charged and discharged during detection cycles. This helps to achieve high sensitivity and fast response times. This type of compensating circuit, however, needs a rather large amount of analog discrete components or requires the development of dedicated integrated circuits, hence increasing manufacturing costs for smoke detectors.
In order to improve the sensitivity of smoke detectors, it is known to modulate the light emitted from the light source and demodulate the output signal of the light receiver correspondingly. These modulation techniques are particularly helpful to differentiate within a smoke detection chamber between ambient light and energy from the light source.
Prior art smoke detectors are described in U.S. Pat. No. 4,206,366 A; U.S. Pat. No. 3,585,621 A; U.S. Pat. No. 4,300,133 A; US 2010/0328085 A1; and US 2010/0302545 A1, for example.